A heater controller is provided in an image forming apparatus such as an electrostatic copier or a laser printer in order to control the temperature of a heater contained in a heater roller. The heater controller controls such heater in the heater roller to become a temperature appropriate for the type of sheet, for example, ordinary sheet, OHP, and the like so that a toner image onto the sheet is thermally fused (fixed) on the sheet while the sheet passing through the heater roller and a pressing roller.
Such kind of heater controller controls a halogen lamp and the like consuming comparatively large power as the heater. Therefore, a larger current fluctuation such as rush current occurs when the conduction state of the heater from an AC power source is switched between ON and OFF. This larger current fluctuation sometimes causes a flicker in the halogen lamp itself or a light emitter which is connected to the same power line as the image forming apparatus.
As an example of countermeasures against such flicker, it is conceivable to continuously control the power supplied to the heater by adopting phase control of interrupting a current in arbitrary phase. However, the waveform of the current flowing into the heater becomes discontinuous rather than a sine wave, and therefore, a harmonic current flows into the power line and terminal noise increases.
JP-A-9-80961 describes a related-art heater wave number controller which performs a wave number control of switching the ON/OFF state of a heater at the period timing at which an AC power supply voltage becomes zero (zero crossing timing), thereby controlling the conduction state of the heater in half-wave units of the AC power supply voltage to suppress occurrence of a harmonic current.
In the heater wave number controller, the switching timing of the conduction state of the heater from OFF to ON is fixed within a control time period set to an integral multiple of a half period of AC power supply voltage and the switching timing from ON to OFF is varied according to the difference between a target temperature and a current heater temperature, thereby controlling the temperature of the heater.
By the way, it is known that a level at which flicker is offensive to a human being is that a change frequency of illuminance becomes about 8.8 times per second (8.8 Hz) although it depends on the illuminance of a light emitter and an individual difference. That is, the higher the switching frequency of the heater conduction state, the shorter a change period of the illuminance (illuminance change period) of a lighting emitter so that the flicker becomes negligible.
However, in the related-art heater wave number controller, if an attempt is made to perform heater control by fractionating the set step of heater output to maintain the accuracy of temperature adjustment, the control time period needs to be long and thus the illuminance change period is prolonged according to the duration of the control time period, and there is a possibility that flicker occurs.